Three-dimensional wordline sharing memory

ABSTRACT

A three dimensional (3D) circuit includes a first layer including at least a first memory cell, a second layer including at least a second memory cell, and a wordline shared by the first memory cell and the second memory cell. The first and second memory cells can be adjacent memory cells or bit cells coupled to different bit lines.

FIELD OF DISCLOSURE

The disclosed circuits and methods relate to semiconductor memories.More particularly, the disclosed circuits and methods relate tothree-dimensional semiconductor memories using wordline sharing.

BACKGROUND

Static random access memories (“SRAM”) include a plurality of cellsdisposed in rows and columns to form an array. SRAM cells include aplurality of transistors coupled to bit lines and wordlines that areused to read and write a bit of data to the memory cell. Intwo-dimensional memories, memory cells and wordlines are in the sameplane. Wordline length increases as word width increases. As a result oflonger wordline length, power consumption increases and overallperformance degrades.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one example of a three-dimensionalsemiconductor integrated circuit with bit cells on different verticallayers sharing the same wordline in accordance with some embodiments.

FIG. 1A is a circuit diagram of bit cells from different vertical layerssharing the same wordline in accordance with some embodiments.

FIG. 2 is a perspective view of a three-dimensional semiconductorintegrated circuit with adjacent bit cells in the same layer and bitcells on different layers sharing the same wordline in accordance withsome embodiments.

FIG. 3 is a perspective view of a three-dimensional semiconductorintegrated circuit with bit cells on different vertical layers sharingthe same wordline similar to FIG. 1 and in accordance with someembodiments.

FIG. 4 is a perspective view of a three-dimensional semiconductorintegrated circuit with bit cells on two different vertical layers belowa shared wordline in accordance with some embodiments.

FIG. 5 is a perspective view of a three-dimensional semiconductorintegrated circuit with bit cells on a predetermined number of differentvertical layers below a shared wordline in accordance with someembodiments.

FIG. 6 is a perspective view of a three-dimensional semiconductorintegrated circuit with bit cells on a predetermined number of differentvertical layers above and below a shared wordline in accordance withsome embodiments.

FIG. 7 is a flow diagram of one example of a method of forming athree-dimensional semiconductor memory in accordance with someembodiments.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description.

The disclosed circuits advantageously reduce issues that can affectconventional two-dimensional semiconductor memories by decreasingresistance and capacitance as a result of reduced wordline lengths.Additionally, the disclosed circuits improves the performance of asemiconductor memory (e.g., power, speed and area).

FIG. 1 illustrates one example of a three-dimensional (“3D”)semiconductor integrated circuit (“IC”) 10. 3D IC 10 includes aplurality of layers such as layers 11 and 12 that are vertically stackedon top of one another in the z-direction. In some embodiments, layers 11and 12 are individual dies that are electrically coupled to one anotherwith at least one through-substrate via (“TSV”) or inter-layer via(“ILV”) (not shown). In some embodiments, 3D IC 10 is a 3D IC such thateach layer 12 is a respective “tier” where each tier includes arespective active device layer and a respective interconnect structure,which can include a plurality of conductive layers (e.g., M1, M2, etc.).Interlayer dielectric (“ILD”) layers (not shown) can be disposed betweendirectly adjacent tiers as will be understood by one of ordinary skillin the art.

3D IC 10 includes an array of bit cells arranged across the two layers11 and 12. Bit cells 15-0 through 15-3 and 17-0 through 17-3 aredisposed on the first layer 11 and bits cells 15-4 through 15-7 and 17-4through 17-7 are disposed on the second layer 12. Bit cells 15-0 through15-3 and 15-4 through 15-7 share the same wordline 13, which is locatedbetween vertically adjacent bit cells in respective layers 11 and 12. Insome embodiments the wordline 13 can be embedded within a portion oflayer 11 and in other embodiments the wordline can be embedded within aportion of layer 12. In some embodiments, the wordline 13 can bedisposed between layers 11 and 12 as a separate layer. Similarly, bitcells 17-0 through 17-3 and 17-4 through 17-7 share the same wordline14, which is located between vertically adjacent bit cells in respectivelayers 11 and 12. The 3D IC further includes a plurality of bit lines(BL[0] through BL[m−1], where m is the number of bit cells in a word).The 3D IC 10 can also include a plurality of complementary bit lines(BLB). Bit lines 16-0 to 16-3 can correspondingly drive bit cells 15-0through 15-3 and/or bit cells 17-0 through 17-3. Bit lines 16-4 to 16-7can correspondingly drive bit cells 15-4 through 15-7 and/or bit cells17-4 through 17-7.

FIG. 1A illustrates further detail of one example of a pair ofvertically stacked bit cells 15-7 and 15-3 as shown in FIG. 1 inaccordance with some embodiments. Bit cell 15-3 is a mirror image of bitcell 15-7, thus for simplicity bit cell 15-3 is described below. Bitcell 15-3 includes a pair of pass transistors 128-1, 128-2 each havingits respective gate coupled to the same wordline 13 at node 130 and itsrespective drain coupled to latch 132. The source of transistor 128-1 iscoupled to a bit line, BL[n] at node 134, and the source of transistor128-2 is coupled to complementary bit line BLB[n] at node 136. Latch 132includes a pair of cross-coupled inverters 138-1 and 138-2, which arearranged such that that output of inverter 138-1 is coupled to the inputof inverter 138-2 at node 124, and the output of inverter 138-2 iscoupled to the input of inverter 138-1 at node 142. Node 142 is alsocoupled to the drain of transistor 128-1.

As shown in FIG. 1A, in some embodiments, inverters 138-1, 138-2 eachinclude a pair of transistors. For example, inverter 138-1 includestransistors 144-1 and 146-1, and inverter 138-2 includes transistors144-2 and 146-2. In some embodiments, transistors 144-1, 144-2 are PMOStransistors and transistors 146-1, 146-2 are NMOS transistors. However,transistors 144-1, 144-2, 146-1, 146-2 can be implemented as othertransistor types.

Transistor 144-1 has its source coupled to a node, which in someembodiments is coupled to a power supply node set at VDD. The drain oftransistor 144-1 is coupled to node 124, and the gate of transistor144-1 is coupled to node 142, which serves as the input of inverter138-1. Transistor 146-1 has its drain coupled to node 124, its sourcecoupled ground or VSS, and is gate coupled to node 142.

Transistor 144-2 of inverter 138-2 has its source coupled to a nodecoupled to VDD, its drain coupled to node 142, and its gate coupled tonode 124. Node 142 is coupled to the drain of transistor 128-1 and isconfigured as the output of inverter 138-2 and the input of inverter138-1. Transistor 146-2 of inverter 138-2 has its source coupled toground or VSS, its drain coupled to node 142, and its gate coupled tonode 124. Although the bit cell 15-3 illustrated in FIG. 1A is shown asa six transistor (“6T”) bit cell, one of ordinary skill in the art willunderstand that bit cell 15-3 (as well as bit cell 15-7) can have otherimplementations with fewer or more transistors. Note, the embodimentsillustrated generally refer to SRAM cells and are not limited thereto.Some embodiments include other types of memories such as DRAM cells thatinclude a plurality of transistors coupled to bit lines and wordlinesthat are used to read and write a bit of data to the memory cell.

FIG. 2 illustrates one example of a 3D IC 20 that includes a pluralityof layers such as layers 21 and 22 that are vertically stacked on top ofone another in the z-direction. In some embodiments, layers 21 and 22are individual dies that are electrically coupled to one another with atleast one through-substrate via (“TSV”) or an inter-layer via (“ILV”) orinter-device via (“IDV”) (not shown). In some embodiments, 3D IC 20 is a3D IC such that each layer 21 and 22 is a respective “tier” where eachtier includes a respective active device layer and a respectiveinterconnect structure, which can include a plurality of conductivelayers (e.g., M1, M2, etc.). Interlayer dielectric (“ILD”) layers (notshown) can be disposed between directly adjacent tiers as will beunderstood by one of ordinary skill in the art.

3D IC 20 includes an array of bit cells arranged across the two layers21 and 22. Bit cells 25-0 through 25-1 and 27-0 through 27-1 aredisposed on the first layer 21 and bits cells 25-2 through 25-3 and 27-2through 27-3 are disposed on the second layer 22 as shown. Bit cells25-0 through 25-3 share the same vertical wordline 23. Similarly, bitcells 27-0 through 27-3 share the same vertical wordline 24. The 3D IC20 further includes a plurality of bit lines. Bit line 26-0 drives bitcells 25-0 and 27-0, Bit line 26-1 drives bit cells 25-1 and 27-1, Bitline 26-2 drives bit cells 25-2 and 27-2, and bit line 26-3correspondingly drive bit cells 25-3 and/or bit 27-3.

FIG. 3 is a perspective view of a three-dimensional semiconductorintegrated circuit 30 with bit cells formed on different vertical layers31 and 32 sharing the same wordlines 36 and 38 and with bit cells on yetother vertical layers 33 and 34 sharing the same wordlines 37 and 39 inaccordance with some embodiments. In this embodiment, adjacent bit cellsresiding on separate stacked layers and residing above and below awordline share the same wordline. For example, bit cells 35-0 through35-3 of layer 31 share the same wordline 36 with corresponding bit cells35-4 through 35-7 of layer 32. Note, in some embodiments the wordline 36can be embedded in layer 31 or in layer 32, as long as wordline 36 iscoupled to corresponding sets of bit cells formed in different layers.Bit cells 35-8 through 35-11 of layer 33 share the same wordline 37 withcorresponding bit cells 35-12 through 35-15 of layer 34. Wordlines 37and 39 similarly share bit cells on an opposing side of the 3D IC 30.The 3D IC 30 further includes a plurality of bit lines. Bit lines 36-0through 36-15 are coupled to cells 35-0 through 35-15 and acorresponding set of bit cells, which are coupled to wordline 38 orwordline 39. This embodiment illustrates the sharing by memory or bitcells across layers above and below (or up and down from in thez-direction) a shared horizontal (x- and y-directions) wordline.

FIG. 4 illustrates a 3D IC 40 with bit cells formed on two differentvertical layers that are positioned below a shared wordline (43 or 44)in accordance with some embodiments. 3D IC 40 includes a plurality oflayers such as layers 41 and 42 that are vertically stacked on top ofone another in the z-direction. In some embodiments as explained above,layers 41 and 42 are individual dies that are electrically coupled toone another with at least one through-substrate via (“TSV”), inter-layervia (“ILV”), or inter-device via (“IDV”) (not shown). In someembodiments, each layer 41 and 42 of 3D IC 40 is a respective “tier”where each tier includes a respective active device layer and arespective interconnect structure, which can include a plurality ofconductive layers (e.g., M1, M2, etc.). Interlayer dielectric (“ILD”)layers (not shown) can be disposed between directly adjacent tiers aswill be understood by one of ordinary skill in the art.

3D IC 40 includes an array of bit cells arranged across the two layers41 and 42. Bit cells 45-0 through 45-3 and 47-0 through 47-3 aredisposed on the first layer 41 and bits cells 45-4 through 45-7 and 47-4through 47-7 are disposed on the second layer 42. Bit cells 45-0 through45-3 and 45-4 through 45-7 correspondingly share the same wordline 43residing above bit cells 45-0 through 45-7. Bit cells 47-0 through 47-7share the same wordline 44, which is disposed above bit cells 47-0through 47-7. In some embodiments, wordlines 43 and 44 can be embeddedin layer 41 (as part of a metal layer M1, M2, M3, etc.) or canalternatively be disposed above layer 41. Bit lines 46-0 to 46-3 arecoupled to bit cells 15-0 through 15-3 and/or bit cells 47-0 through47-3. Bit lines 46-4 to 46-7 are coupled to bit cells 45-4 through 45-7and/or bit cells 47-4 through 47-7.

FIG. 5 illustrates a 3D IC 50 with bit cells on a number, X, ofdifferent vertical layers with the bit cells being positioned below ashared wordline (58 or 59) in accordance with some embodiments. 3D IC 50includes a plurality of layers such as layers 51-1, 51-2 to 51-X thatare vertically stacked on top of one another in the z-direction. In someembodiments as explained above, layers 51-1, 51-2 to 51-X are individualdies that are electrically coupled to one another with at least one TSV,ILV, or IDV (not shown). In some embodiments, 3D IC 50 is a 3D IC suchthat each layer 51-1, 51-2 to 51-X is a respective “tier” where eachtier includes a respective active device layer and a respectiveinterconnect structure, which can include a plurality of conductivelayers (e.g., M1, M2, etc.). ILD layers (not shown) can be disposedbetween directly adjacent tiers as will be understood by one of ordinaryskill in the art.

3D IC 50 includes an array of bit cells arranged across the X number oflayers below the wordlines 58 and 59. Bit cells 55-0 through 55-3 and57-0 through 57-3 are disposed on the first layer 51-1, bits cells 55-4through 55-7 and 57-4 through 57-7 are disposed on the second layer51-2, and bits cells 55-8 through 55-11 and 57-8 (not shown) through57-11 are disposed on the Xth layer 51-X. Bit cells 55-0 through 55-3,55-4 through 55-7 and 55-8 through 55-11 correspondingly share the samewordline 58 residing above bit cells 55-0 through 55-11. Bit cells 57-0through 57-3, 57-4 through 57-7, and 57-8 through 57-11 correspondinglyshare the same wordline 59 residing above bit cells 57-0 through 57-11.In some embodiments, wordlines 58 and 59 can be embedded in layer 51-1or can alternatively be disposed above layer 51-1. Bit lines 56-0 to56-3 can correspondingly drive or are driven by bit cells 55-0 through55-3 and/or bit cells 57-0 through 57-3. Bit lines 56-4 to 56-7 cancorrespondingly drive bit cells 55-4 through 55-7 and/or bit cells 57-4through 57-7. Bit lines 56-8 to 56-11 can correspondingly drive or bedriven by bit cells 55-8 through 55-11 and/or bit cells 57-8 through57-11.

FIG. 6 illustrates a 3D IC 60 with bit cells on a number, X, ofdifferent vertical layers below a shared wordline and with bit cells ona number, Y, of different vertical layers above the shared wordline inaccordance with some embodiments. 3D IC 60 includes a plurality oflayers such as layers 61-1 to 61-X that are vertically stacked on top ofone another in the z-direction below the shared wordline and a pluralityof layers such as layers 62-1 to 62-Y vertically stacked above theshared wordline. In some embodiments as explained above, layers 61-1 to61-X and 62-1 to 62-Y are individual dies that are electrically coupledto one another with at least one TSV, TDV, or ILV (not shown). In someembodiments, each layer of 3D IC 60 is a respective “tier” where eachtier includes a respective active device layer and a respectiveinterconnect structure, which can include a plurality of conductivelayers (e.g., M1, M2, etc.). ILD layers (not shown) can be disposedbetween directly adjacent tiers as will be understood by one of ordinaryskill in the art.

3D IC 60 includes an array of bit cells arranged across the X number oflayers below the wordlines 63-1 and 63-2 and across the Y number oflayers above the wordlines 63-1 and 63-2. Bit cells 65-0 through 65-3coupled to wordline 63-1 and another corresponding set of bit cellscoupled to wordline 63-2 are disposed on layer 61-1, bits cells 65-4through 65-7 coupled to wordline 63-1 and another corresponding set ofbit cells coupled to wordline 63-2 are disposed on layer 61-X. Bit cells64-0 through 64-3 coupled to wordline 63-1 and another corresponding setof bit cells coupled to wordline 63-2 are disposed on layer 62-1, bitscells 64-4 through 64-7 coupled to wordline 63-1 and anothercorresponding set of bit cells coupled to wordline 63-2 are disposed onlayer 62-Y.

Bit cells 65-0 through 65-3 and bit cells 65-4 through 65-7correspondingly share the same wordline 63-1 residing above bit cells65-0 through 65-7. Furthermore, bit cells 64-0 through 64-3 and bitcells 64-4 through 64-7 correspondingly share the same wordline 66-0residing below bit cells 64-0 through 64-7. Another set of bit cellscorrespondingly share wordline 63-2 in a similar arrangement. 3D IC 60further includes a number of bit lines. Bit lines 66-0 to 66-3 cancorrespondingly drive bit cells 65-0 through 65-3 to store a logic zeroor a logic one in a storage node 124, 142 (FIG. 1A) or can be coupled tostorage nodes 124, 142 to read a logic zero or a logic one from one ofthe storage nodes. Bit lines 66-4 to 66-7 (FIG. 6) can correspondinglydrive bit cells 65-4 through 65-7 to store a logic zero or a logic onein storage nodes 124, 142 (FIG. 1A) or can be coupled to storage nodes124, 142 to read a logic zero or a logic one from the storage nodes. Bitlines 67-0 to 67-3 (FIG. 6) can correspondingly drive bit cells 64-0through 64-3 to store a logic zero or a logic one in storage nodes 124,142 (FIG. 1A) or can be coupled to storage nodes 124, 142 to read alogic zero or a logic one from the storage nodes, and bit lines 67-4 to67-7 (FIG. 6) can correspondingly drive bit cells 64-4 through 64-7 tostore a logic zero or a logic one in storage nodes 124, 142 (FIG. 1A) orcan be coupled to storage nodes 124, 142 to read a logic zero or a logicone from the storage nodes.

FIG. 7 is a flow diagram of one example of a method 70 of manufacturinga three-dimensional semiconductor memory in accordance with someembodiments. At block 72, circuitry is formed in and/or on a firstlayer. For example, the active devices of the bit or memory cells (e.g.,transistors 128, 128, 144,146) of a bit cell array are formed in asemiconductor substrate (not shown) using a semiconductor processingtechnology. In some embodiments, the bit cells are formed usingcomplementary metal oxide semiconductor (“CMOS”) semiconductorprocessing technology.

At block 74, circuitry is formed in and/or on the second layer. Forexample, the active devices including a second bit or memory cell or setof memory or bit (e.g., transistors 128, 128, 144,146) cell are formedin another semiconductor substrate (not shown) using a semiconductorprocessing technology. In some embodiments, the bit cells or memorycells are formed using CMOS semiconductor processing technology.

At block 75, the first and second layers are coupled together. Forexample, if first and second layers 11 and 12 of FIG. 1 are separatesemiconductor chips, then layers 11 and 12 are vertically stacked on topof one another, aligned, and bonded together. In some embodiments, suchas embodiments in which layers 11 and 12 are tiers, the layers arestacked on top of one another to create a 3D stacked CMOS IC. One ofordinary skill in the art will understand that one or more layers can bedisposed between layers 11 and 12 in some embodiments.

At block 76, part of the circuitry formed in either the first layer orthe second layer includes a wordline of the word which is used to sharethe first memory cell and the second memory cell where the first memorycell and the second memory cell are formed on separate layers. In someembodiments, the wordline and other conductive lines are embedded inmetal layers (e.g., M1, M2, etc.) in either layer 11 or layer 12(FIG. 1) or formed between layers 11 and 12 to couple respective bitcells via a wordline 13 or worldine 14 (FIG. 1). In some embodiments,the conductive lines are TSVs or ILVs.

The disclosed circuits and methods described herein advantageouslyreduce issues that can affect conventional two-dimensional semiconductormemories by decreasing resistance and capacitance as a result of reducedwordline lengths. The implementation of a stacked bit cell array sharingwordlines further improves area efficiency. Additionally, the disclosedcircuits and methods improves the performance of a semiconductor memoryin terms of power and speed.

In some embodiments, a 3D IC includes a first layer including at least afirst memory cell, a second layer including at least a second memorycell, and a wordline shared by the first memory cell and the secondmemory cell. The second layer disposed in a vertical stack with thefirst layer.

In some embodiments the first memory cell and the second memory cell arehorizontally adjacent memory cells coupled to different bit lines.

In some embodiments the first memory cell and the second memory cell arevertically adjacent memory cells coupled to different bit lines. In someembodiments the different bit lines include a first bit line in thefirst layer and a second bit line in the second layer.

In some embodiments the first layer includes a first plurality of memorycells, the second layer includes a second plurality of memory cells, andthe wordline is shared by the first plurality of memory cells and thesecond plurality of memory cells. In some embodiments, the wordlineextends horizontally across one of the first layer or the second layerand is shared with the first plurality of memory cells disposedvertically above the wordline and with the second plurality of memorycells disposed vertically below the wordline.

In some embodiments, the wordline extends horizontally across one of thefirst layer or the second layer and is shared with the first pluralityof memory cells and the second plurality of memory cells both disposedvertically below the wordline.

In some embodiments, the 3D circuit includes a first plurality of memorycells disposed in the first layer, a second plurality of memory cellsdisposed in the second layer, and respective pluralities of memory cellsdisposed in adjacent respective layers below the first and secondlayers, wherein the wordline extends horizontally across and above thefirst plurality of memory cells and is shared with the first pluralityof memory cells, the second plurality of memory cells, and therespective pluralities of memory cells all disposed vertically below thewordline.

In some embodiments, the 3D circuit further includes a first pluralityof memory cells disposed above the first layer in respective layersabove the first layer, and a second plurality of memory cells disposedbelow the second layer in respective layers below the second layer,wherein the wordline is a horizontal wordline disposed between the firstlayer and the second layer and the horizontal wordline is shared withthe first plurality of memory cells and the second plurality of memorycells.

In some embodiments, the first memory cell and the second memory cellsare adjacent static random access memory cells.

In some embodiments, the wordline extends vertically from the firstlayer to the second layer and is shared among memory cells on separatelayers with memory cells including the first memory cell disposedhorizontally to the left of the wordline and memory cells disposedhorizontally to the right of the wordline.

In some embodiments, a semiconductor memory includes a bit cell arrayincluding a first memory cell of the bit cell array disposed in a firstlayer and a second memory cell of the bit cell array disposed in asecond layer. A wordline is shared by the first memory cell and thesecond memory cell of the bit cell array. The first and second layersare arranged in a vertical stack.

In some embodiments, the first memory cell and the second memory cellare vertically adjacent memory cells coupled to different bit lines. Insome embodiments, the different bit lines include a first bit line inthe first layer and a second bit line in the second layer.

In some embodiments, a method includes the steps of forming circuitry ona first layer, the circuitry on the first layer including a first memorycell, forming circuitry on a second layer, the circuitry on the secondlayer including a second memory cell, and forming a wordline such thatthe wordline is share by the first memory cell and the second memorycell .

In some embodiments, the first memory cell and the second memory cellsare formed as horizontally adjacent memory cells coupled to differentbit lines. In some embodiments, the first memory cell and the secondmemory cell are formed as vertically adjacent memory cells coupled todifferent bit lines.

In some embodiments, the method further includes coupling the firstlayer and the second layer together.

In some embodiments, the method includes coupling the first memory celland the second memory cell via the wordline using through-silicon-vias.

In some embodiments, the first layer includes a first plurality ofmemory cells for the word, the second layer includes a second pluralityof memory cells for the word, and the wordline for the word is shared bythe first plurality of memory cells and the second plurality of memorycells.

Although the circuits and methods have been described in terms ofexemplary embodiments, they are not limited thereto. Rather, theappended claims should be construed broadly, to include other variantsand embodiments of the circuits and methods, which may be made by thoseskilled in the art without departing from the scope and range ofequivalents of the circuits and methods.

What is claimed is:
 1. A three dimensional (3D) circuit, comprising: afirst layer including at least a first memory cell; a second layerincluding at least a second memory cell, the second layer disposed in avertical stack with the first layer; and a wordline shared by the firstmemory cell and the second memory cell.
 2. The 3D circuit of claim 1,wherein the first memory cell and the second memory cell arehorizontally adjacent memory cells coupled to different bit lines. 3.The 3D circuit of claim 1, wherein the first memory cell and the secondmemory cell are vertically adjacent memory cells coupled to differentbit lines.
 4. The 3D circuit of claim 3, wherein the different bit linesinclude a first bit line in the first layer and a second bit line in thesecond layer.
 5. The 3D circuit of claim 1, wherein the first layerincludes a first plurality of memory cells, the second layer includes asecond plurality of memory cells, and the wordline is shared by thefirst plurality of memory cells and the second plurality of memorycells.
 6. The 3D circuit of claim 5, wherein the wordline extendshorizontally across one of the first layer or the second layer and isshared with the first plurality of memory cells disposed verticallyabove the wordline and with the second plurality of memory cellsdisposed vertically below the wordline.
 7. The 3D circuit of claim 5,wherein the wordline extends horizontally across one of the first layeror the second layer and is shared with the first plurality of memorycells and the second plurality of memory cells both disposed verticallybelow the wordline.
 8. The 3D circuit of claim 1, further comprising afirst plurality of memory cells disposed in the first layer, a secondplurality of memory cells disposed in the second layer, and respectivepluralities of memory cells disposed in adjacent respective layers belowthe first and second layers, wherein the wordline extends horizontallyacross and above the first plurality of memory cells and is shared withthe first plurality of memory cells, the second plurality of memorycells, and the respective pluralities of memory cells all disposedvertically below the wordline.
 9. The 3D circuit of claim 1, furthercomprising a first plurality of memory cells disposed above the firstlayer in respective layers above the first layer, and a second pluralityof memory cells disposed below the second layer in respective layersbelow the second layer, wherein the wordline is a horizontal wordlinedisposed between the first layer and the second layer and the horizontalwordline is shared with the first plurality of memory cells and thesecond plurality of memory cells.
 10. The 3D circuit of claim 1, whereinthe first memory cell and the second memory cells are adjacent staticrandom access memory cells.
 11. The 3D circuit of claim 1, wherein thewordline extends vertically from the first layer to the second layer andis shared among memory cells on separate layers with memory cellsincluding the first memory cell disposed horizontally to the left of thewordline and memory cells including the second memory cell disposedhorizontally to the right of the wordline.
 12. A semiconductor memory,comprising: a bit cell array, comprising: a first memory cell of the bitcell array disposed in a first layer; a second memory cell of the bitcell array disposed in a second layer, the second layer disposed in avertical stack with the first layer; and a wordline shared by the firstmemory cell and the second memory cell of the bit cell array.
 13. Thesemiconductor memory of claim 12, wherein the first memory cell and thesecond memory cell are vertically adjacent memory cells coupled todifferent bit lines.
 14. The semiconductor memory of claim 13, whereinthe different bit lines include a first bit line in the first layer anda second bit line in the second layer.
 15. A method, comprising: formingcircuitry on a first layer, the circuitry on the first layer including afirst memory cell; forming circuitry on a second layer, the circuitry onthe second layer including a second memory cell; and forming a wordlinesuch that the wordline is shared by the first memory cell and the secondmemory cell.
 16. The method of claim 15, wherein the first memory celland the second memory cells are formed as horizontally adjacent memorycells coupled to different bit lines.
 17. The method of claim 16,wherein the first memory cell and the second memory cell are formed asvertically adjacent memory cells coupled to different bit lines.
 18. Themethod of claim 15, wherein the method further comprises coupling thefirst layer and the second layer together.
 19. The method of claim 15,wherein the method comprises coupling the first memory cell and thesecond memory cell via the wordline using through-silicon-vias.
 20. Themethod of claim 15, wherein the first layer includes a first pluralityof memory cells for the word, the second layer includes a secondplurality of memory cells for the word, and the wordline for the word isshared by the first plurality of memory cells and the second pluralityof memory cells.